According to Greek legend, Thessalonike, half-sister of Alexander the Great, metamorphosed into a mermaid. "Is Alexander still alive?" she would ask sailors after her brother's death. "Yes, he is alive and in power over the world." Right answer! These words granted safe passage across the seas, and, closer to home, the threshold of a 1970s Oxford society of lustful "mermen", known as Red Tents. While others were exploring the eccentricities of this half-imagined Oxford haunt, Peter Stothard was making his fourth, somewhat more salubrious foray into the world of Alexandria. His lifelong ambition was to write a book about its greatest queen, Cleopatra.

Forty years later and we have the fruit of that ambition, Alexandria: The Last Nights of Cleopatra, Stothard's eighth attempt and first result. It pays homage to a topic that has touched every part of his life to date, his childhood, student days at Oxford, his experience as editor of The Times and Times Literary Supplement, and more besides. It is a book concerned as much — or as Stothard himself often pauses to observe, more — with his life as with his life's obsession, the strong-willed seductress herself.

Cleopatra was one in a line of Macedonians who ruled Egypt after it submitted to a general of Alexander the Great (the son, in fact, of Alexander's father's mistress). Her father was King Ptolemy XII "the Flautist", who famously established a relationship with Rome to secure his throne. The relationship between the two centres set an important precedent for the future ruler.

So much of the luscious detail that surrounds Cleopatra's life and rule is the stuff of fiction that Stothard is justified in giving it short shrift. Why, he shrugs, would a highly intelligent woman rely on an asp to inject her with death when she is surrounded by men who are expert in administering painlessness?

"Biographers want the dead but they have only the living. We must work with the tools we have," Stothard remarks. Which tools are these? There are the ramblings of historians often too young to remember the events they describe, the dialogue of great playwrights, the snippets of context and detail, such as what people might have eaten and why (Stothard is particularly good on this), but the gaping lacunae which inhabit all ancient lives are often the biographer's most useful tools.